Fortnite is set in contemporary Earth, where the sudden appearance of a worldwide storm causes 98% of the world's population to disappear, and zombie-like creatures rise to attack the remainder. Considered by Epic as a cross between Minecraft and Left 4 Dead, Fortnite has up to four players cooperating on various missions on randomly-generated maps to collect resources, build fortifications around defensive objectives that are meant to help fight the storm and protect survivors, and construct weapons and traps to engage in combat with waves of these creatures that attempt to destroy the objectives. Players gain rewards through these missions to improve their hero characters, support teams, and arsenal of weapon and trap schematics to be able to take on more difficult missions. The game is supported through microtransactions to purchase in-game currency that can be used towards these upgrades.

One day, 98% of Earth's population suddenly disappeared, and the remaining population found the skies covered in dense clouds, creating chaotic storms that dropped husks, humanoid zombie-like creatures, that attacked the living. The survivors found ways to construct "storm shields", a field that cleared the storm clouds from immediately overhead and reduced the attacks from husks, and used these to set up survivor bases across the globe. The player is a commander of one of these bases, charged with going out of the storm shield to find resources, survivors, and other allies to help expand their storm shield and find a way to return Earth to its normal state.